Walnut rootstock ‘RX1’

ABSTRACT

A new and distinct variety of walnut rootstock denominated ‘RX1’ is described. This new variety, ‘RX1’, can be propagated through standard tissue culture micropropagation. It has excellent survivability in the nursery and orchard. The new variety also has reduced susceptibility to damage from  Phytophthora citricola  in greenhouse screens and in the field compared to other available walnut rootstocks.

FIELD OF THE INVENTION

Botanical/commercial classification: Juglans microcarpa×Juglansregia/new walnut rootstock. Varietal denomination: ‘RX1’.

BACKGROUND OF THE INVENTION

The present invention relates to a new and distinct clonal rootstock forEnglish walnut (Juglans regia) that has been denominated varietally as‘RX1’, and more particularly to such a walnut rootstock that has reducedsusceptibility to cankering by Phytophthora (Phytophthora citricola),and that further is easily clonally propagated by micropropagation.

It has long been recognized that Phytophthora root and crown rots aresome of the most serious diseases of walnut worldwide. In California,Phytophthora citricola and P. cinnamomi are recognized as the mostvirulent species of the fungus, but P. citricola is more widespread. Therootstock of the present invention, ‘RX1’, has been identified as beingmore resistant to P. citricola than other available clonal walnut(Juglans) rootstocks.

SUMMARY OF THE INVENTION

It was found that the walnut rootstock ‘RX1’ of the present inventionexhibits the following combination of characteristics:

-   -   a) can be propagated through standard tissue culture        micropropagation;    -   b) has excellent survivability in the nursery and orchard; and    -   c) has reduced susceptibility to damage from Phytophthora        citricola in greenhouse screens and in the field compared to        other available walnut rootstocks.

BRIEF DESCRIPTION OF THE TABLES

Table 1 shows comparative nursery performance of ‘RX1’ and otherrootstock clones grown in Stanislaus County, Calif. in 2004.

Table 2 shows comparative nursery performance of ‘RX1’ and otherrootstock clones grown at in Butte County, Calif. in 2004.

Table 3 shows field performance of ‘RX1’ and other clonal and seedlingrootstocks in non-infested soil and soil infested with Phytophthoracitricola.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows relative susceptibility of ‘RX1’ and two other potentialrootstock clones to Phytophthora citricola, and the effect ofpre-inoculation chilling on disease severity, 2003 greenhouse screen.

FIG. 2 shows relative susceptibility of ‘RX1’ and two other potentialrootstock clones to Phytophthora citricola, data combined for plantssubjected to pre-inoculation chilling and non-chilled plants, 2003greenhouse screen.

FIG. 3 shows relative susceptibility of ‘RX1’ and six other potentialrootstock clones to Phytophthora citricola, 2003 greenhouse screen.

FIG. 4 shows relative susceptibility of 10 hybrid walnut clones toPhytophthora citricola, 2004 greenhouse screen.

FIG. 5 shows relative susceptibility of 17 hybrid walnut clones andNorthern California black walnut to Phytophthora citricola, 2006greenhouse screen.

FIG. 6 shows grafted ‘RX1’ in a new orchard.

FIG. 7 shows ‘RX1’ in Phytophthora field trial.

FIG. 8 shows grafted ‘RX1’ in replant situation.

FIG. 9 shows visual rating of tree growth and condition of clonal andseedling test trees at a California field site in 2006.

FIG. 10 shows percent mortality for clonal selections and otherrootstocks at a California field site.

FIG. 11 shows percent mortality for clonal selections and otherrootstocks at a California field site.

FIG. 12 shows bark and new leaves of three-year old ‘RX1’ tree.

FIG. 13 shows greenhouse grown ‘RX1’ tree about 6 months old.

FIG. 14 shows upper side of leaf of ‘RX1’.

FIG. 15 shows lower side of leaf of ‘RX1’.

FIG. 16 shows the flower of ‘RX1’.

DETAILED DESCRIPTION OF THE INVENTION

The new rootstock, ‘RX1’ was selected as part of the “Paradox DiversityStudy” (PDS) which was initiated in 1996 to study the genetic diversityof commercial walnut rootstocks. The hybrid of J. hindsii×J. regia,commonly known as ‘Paradox’ (not patented), is the most frequentlyplanted rootstock for English walnut in California. The study includedapproximately 300–500 seed (depending on the predicted percent‘Paradox’), from 37 black walnut sources of ‘Paradox’ supplied byCalifornia walnut nurseries, and 7 controlled crosses andopen-pollinated controls from several different walnut species includingTexas black, Juglans microcarpa. Seed or seedlings were distributed tocooperating researchers for tests of response to nematodes (Pratylenchusvulnus), Phytophthora (seed supplied), crown gall (Agrobacteriumtumefaciens) and the orchard environment (field trials). The study wasrepeated in 1997.

In fall of 1997, seed from a Juglans microcarpa designated as DJUG 29.11in location B6-3 at the National Germplasm Repository, Davis and growingin Winters, Calif. was tested against Phytophthora citricola. Fromresults of the previous year, about 50% germination and about 50%hybrids with J. regia from this tree was expected. In fact in 1998,germination was better (70%) but percent hybrids were very low (5%). Dueto lack of sufficient seedlings for screening, a representative of theJ. microcarpa×J. regia hybrid family was asexually reproduced bystandard tissue culture micropropagation in Davis, Calif. The seedlingchosen (98-RX-SD8) later became ‘RX1’. Thus, ‘RX1’ originated as asingle plant. It was introduced into culture in summer 1998 using thestandard tissue culture micropropagation protocol. In fall 1998, thecultures were transferred to a nursery for further multiplication androoting.

In summer 2001, a replicated trial in the greenhouse to determine therelative susceptibility of ‘RX1’ to Phytophthora citricola wasinitiated. The clone appeared to have resistance to the pathogen inpreliminary tests. In September, 2001, the clone was evaluated forfurther production of plants. Between 2001 and 2005, ‘RX1’ wasmultiplied, rooted and acclimatized for trials for response toPhytophthora citricola and for additional field trials. During summers2002, 2003, 2004, and 2006, plants were transplanted into appropriatecontainers, grown to appropriate size for screening, grown onappropriate inocula, and subjected to repeated greenhouse experiments toevaluate resistance of ‘RX1’ and other selected clones to P. citricola .Several modifications in propagation and pre-inoculation treatments weremade including induction of dormancy of plants and treatment withhormones. ‘RX1’ was consistently at least moderately resistant to thepathogen (FIGS. 1–4).

In August 2006 a screen for P. citricola response was conducted withplants of ‘RX1’ that had been through cycles of dormancy which tended toequalize growth and kept them small enough to facilitate mass screening.The cycles included dormancy induced by storage at 6C for 3–5 months(2004), growth in the greenhouse for one year (2005) and naturaldormancy in a lath house followed by growth in the greenhouse (2006).The screen for resistance was initiated in August 2006 by transplantingindividual plants from one-liter pots to two-liter pots filled withpotting mix soil that was either artificially infested with P. citricola(45 ml of P. citricola-infested V8 juice-oat-vermiculite substrate perliter of soil) or treated as a control (45 ml sterile substrate perliter of soil). There were 5 replicate plants planted in non-infestedsoil and 10–20 replicate plants in infested soil, evenly distributed ina split-plot design (main plots were inoculum treatments, subplots wererootstock) among 5 blocks. Every two weeks after transplanting, the soilin each pot was flooded for 48 hours. Three months after transplanting,the root and crown systems were washed free from soil and evaluatedvisually for incidence and severity of crown and root rot. Among the 17clonal hybrids evaluated in the screen, ‘RX1’ was one of the hybridsmost resistant to P. citricola (FIG. 5).

During the propagation of plants for Phytophthora testing, plants werealso being propagated for field trials. These were grown at twonurseries in 2004. ‘RX1’ was one of the smallest plants at bothnurseries (Tables 1 and 2), but produced between 70% and 75% graftablerootstocks as determined by the nursery. These were either grafted inplace with ‘Chandler’ (U.S. Plant Pat. No. 4,388) or distributed forgrafted field trials for replant situations or Phytophthora fieldscreening in 2005. The ‘Chandler’-grafted ‘RX1’ (n=80) was planted in anew orchard with another promising clone AZ2 (n=80) and seedling‘Paradox’ (J. hindsii×J. regia) provided by the nursery. AZ2 turned outto be a weak clone that could not be transplanted bare root, andsurvival was very poor after transplanting. Nearly all the ‘RX1’survived and were indistinguishable from the seedling ‘Paradox’ (FIG.6).

For the Phytopthora field trial, 30 each of 11 different genotypesincluding ‘RX1’ were planted in May, 2005 in Davis, Calif. and wereartificially inoculated with Phytophthora citricola. A randomized blocksplit plot design was used. For each rootstock clone, there were sixfour-tree plots to be infested and six single tree plots to serve asuninoculated controls. Northern California black (J. hindsii) andwingnut (Pterocarya stenoptera) were included as susceptible andresistant controls, respectively. In January 2006, 100 ml of a V8juice-oat mixture infested with P. citricola was mixed into the upper 5cm of soil around the trunk of each tree. A sterile mixture was appliedto the uninoculated controls.

The block artificially inoculated with Phytophthora was assessed forgrowth in trunk circumference and development of crown rot as indicatedby trunk cankers extending up from the soil surface in November 2006.Sixty-two percent of the susceptible controls were rotted or dead. ‘RX1’was one of the smaller clones (Table 3), but it was thriving (FIG. 7)and not affected by the inoculation (Table 3).

Preliminary results from grafted field trials suggest that ‘RX1’ is asurvivor in spite of the challenge of being in replant sites (FIGS.8–11).

BOTANICAL DESCRIPTION OF THE PLANT

This description is based on a 6-month old greenhouse grown clone of‘RX1’ produced through standard tissue culture micropropagation, a3-year old ‘RX1’ in the Phytophthora field screen and a 2-year old RX1growing in Davis, Calif. Data for the botanical description werecollected in spring, 2007.

The Munsell Color Charts for Plant Tissues (1977. GretagMacbeth, NewWindsor, N.Y.) is used in the identification of color. Also, commoncolor terms are to be accorded their ordinary dictionary significance.

-   Botanical classification: Juglans microcarpa×Juglans regia.-   Female parent: Juglans microcarpa-   Male parent: Juglans regia

The male parent is identified to be of the species J. regia, or Englishwalnut. J. regia typically has 7–9 leaflets while J. microcarpa, thefemale patent, typically has 15–23 leaflets. ‘RX1’ differs from itsfemale parent by having fewer leaflets/leaf, broader leaflets and morevigor. ‘RX1’ differs from its male parent by having more leaflets/leafand narrower leaflets.

-   Plant: The growth habit of the tree is illustrated in FIG. 7. This    3-year old tree is approximately 3.05 meters tall. Bark of two-year    old wood is dark brown (2.5Y 5/2). Bark color of one-year old wood    is lighter and redder (7.5YR 5/4) (FIG. 12). Lenticels, about 48 in    one square cm, are buff-colored (7.5YR 8/2). The six month old,    greenhouse-grown tree is about 45 cm tall with a stem diameter of    about 0.8 cm (FIG. 13). The stem is green (5GY 5/10) with scattered    lenticels (2.5Y 8/4) more dense towards the base and about 0.5 mm    long.-   Trunk diameter: ‘RX1’ is 6.1 meters in height and 11 cm diameter DBH    at four years of age.-   Foliage: The leaves are pinnately compound and alternate. The    slightly pubescent new spring foliage (FIG. 12) has reddish new    leaves (10R 5/8) and green older leaves (5GY 5/6). There are 13–15    leaflets. The six-month old greenhouse-grown tree has fewer leaflets    (9–11). Leaves are 30 cm long and 28–30 cm wide with petioles 5–8 cm    long. Leaflets are 12–14 cm long and 5–7 cm wide, dark green on the    upper surface (5GY 5/10) (FIG. 14) and slightly lighter on the lower    surface (5GY 7/4) (FIG. 15). Leaflet margins are entire i.e. no    serration. The pubescence on young, unfolding leaves is found on the    adaxial and abaxial surfaces as well as on the rachis. The mature    leaves are not pubescent and are very smooth. The venation is    pinnate.-   Inflorescence: The flowers are small (2 mm×5 mm) and borne in two or    three at the shoot tip (FIG. 16). The stigma surface is red (5R 5/8)    and the involucre is green (2.5GY 6/6) covered with sticky hairs.    There is no calyx. ‘RX1’ produces a light crop of nuts.-   Disease resistance and susceptibility: This rootstock is more    resistant to Phytophthora citricola in greenhouse tests than other    Juglans rootstocks. It is the most resistant variety to P. citricola    known to the inventors.-   Usage: The new rootstock of the present invention provides walnut    growers with a new clonally propagated rootstock. It can be easily    micropropagated through standard tissure culture micropropagation.

TABLE 1 Clones grown in Stanislaus County, California in 2004 PlantedGraft- Graftable Diameter (mm) Clone N able N % Mean SD Range CVNematodes VX211 106 87 82 31 4.9 21-44 12.6 Phytophthora AZ2 230 151 6626 5 13-38 19.2 AZ3 49 24 49 25 6.7 11-37 26.8 NZ1 172 111 64 26 4.410-39 16.9 JX2 246 191 78 29 4.1 13-39 14.1 RX1 104 78 75 18 1.6 14-228.8 AX1 163 86 53 27 4.3 14-40 15.9 GZ1 108 83 77 26 5.4 13-40 20.8 Px1247 154 62 26 4.6 12-40 17.7 AZ1 52 38 73 30 4.4 22-43 14.7 UX1 27 23 8525 4 15-30 16 GZ2 47 38 81 26 4.5 15-33 17.3 Blackline WIP3 158 66 42 265 12-35 19.2 WIP9 10 6 60 25 2.3 23-99 9.2 Control UX022 71 59 83 23 3.714-29 16.1 English Vina 14 10 71 18 3.7 13-24 20.5 Sunland 64 20 31 263.8 18-31 14.6 Totals 1868 1225 66 25

TABLE 2 Clones grown in Butte County, California in 2004. PlantedGraftable Graftable Diameter (mm) Clone N N % Mean SD Range CV AX1 120107 89 19 4.6 10-30 26 AZ2 120 102 85 21 4.7 10-31 22 RX1 120 84 70 193.2 10-27 17 Totals 360 293 81 20

TABLE 3 Field performance of clonal Paradox hybrids, Northern Californiablack walnut, and Chinese wingnut rootstocks in non-infested soil andsoil infested with Phytophthora citricola, Davis. Maternal backgroundIncidence of Clone of hybrid Soil treatment crown (or species) (orspecies of standard) (January 2006) rot (%) AX1 californica Control 0 cP. citricola 4 c AZ2 (major x hindsii)x nigra Control 0 c P. citricola 0c NZ1 (major x hindsii)x nigra Control 0 c P. citricola 0 c GZ1 hindsiiControl 0 c P. citricola 4 c JX2 hindsii Control 0 c P. citricola 0 cPX1 hindsii Control 0 c P. citricola 8 c VX211 hindsii Control 0 c P.citricola 0 c RX1 microcarpa Control 0 c P. citricola 0 c WIP3 hindsii xregia Control 0 c P. citricola  8 bc (NCB) (J. hindsii) Control 16 b  P.citricola 62 a  (Wingnut) (Pt. stenopiera) Control 0 c P. citricola 0 cPercent of Incidence of Increase in Clone trunk circ. tree mortalitytrunk circ. (or species) Necrotic % (mm) AX1 0 c 0 c 163 c 1 c 0 c 146cde AZ2 0 c 0 c 116 fg 0 c 0 c 117 fg NZ1 0 c 0 c 116 fg 0 c 0 c 130 defGZ1 0 c 0 c 157 cd 1 c 0 c 150 cd JX2 0 c 0 c 166 bc 0 c 0 c 135 def PX10 c 0 c 169 bc 1 c 0 c 157 cd VX211 0 c 0 c 191 b 0 c 0 c 147 cde RX1 0c 0 c 112 fg 0 c 0 c 116 fg WIP3 0 c 0 c 100 g 2 c 0 c 121 efg (NCB) 17b  17 b  68 h 59 a  59 a  57 h (Wingnut) 0 b 0 e 226 a 0 b 0 c 193 b²All trees were planted May 2005. The assements of crown rot andmortality were made Nov. 21, 2006. Means within a column and withoutletters in common are significantly different (Waller k ratio).

1. A new and distinct variety of walnut rootstock plant designated ‘RX1’as shown and described herein.